ABSTRACT: The mechanisms responsible for hypoxic contractile failure and reoxygenation injury in the myocardium, will be studied in a single-myocyte hypoxia model. Single isolated ventricular myocytes from rats, rabbits and guinea-pigs will be subjected to anoxia or graded hypoxia (1-3 torr) using the Laminar Counterflow Barrier Well, a novel device developed by the Principal Investigator, which permits, for the first time, open access to the cell for electrophysiologic studies during profound hypoxia. Myocytes will be voltage clamped by the "whole cell" patch pipette technique during hypoxia and reoxygenation, with simultaneous monitoring of contraction by video edge tracking, and of cytosolic calcium transients using strobe-excited indo-1 fluorescence. Sarcoplasmic reticulum calcium stores will be estimated by rapid superfusion of caffeine from a micropipette in indo-1 loaded cells. The importance of action potential narrowing (due to outward currents), depression of the slow inward calcium current and alteration of myofilament sensitivity in producing rapid hypoxic contractile failure will be determined by measuring the calcium transient and contraction amplitude when hypoxic outward currents are overcome by voltage clamp and blockade with Tolbutamide. Repeated episodes of hypoxic contractile failure, followed by reoxygenation before contracture, will be used to determine if myocardial "stunning" exists at the single cell level; if so, it will be determined whether alteration of the calcium transient and/or sarcolemmal currents are responsible. The mechanism of "early" calcium overload, which was found in pilot studies to occur during graded hypoxia, but not anoxia, will be studied. The source of the rise in cytosolic calcium during the hypoxic contracture will be determined by varying the ionic milieu and applying caffeine, ryanodine and slow channel blockers, in indo-1 loaded myocytes in the contracted (rigor) state. Reoxygenation after contracture causes rounding up (oxygen paradox) of myocytes, with a probability which increases with the duration of rigor. The contributions of calcium accumulation, sodium accumulation, sarcolemmal injury and adenine nucleotide depletion to this process will be explored by manipulating the extracellular and intracellular environment during rigor and reoxygenation.